This invention relates to a semiconductor memory system which uses an electrically rewritable nonvolatile memory, and more particularly, to a method of implementing a snapshot function of memory contents in a storage system which uses a flash memory as a nonvolatile memory.
A semiconductor memory system which is equipped with a flash memory and has a memory control function of preventing execution of a deletion cycle for each change of data stored in the flash memory has been known (e.g., U.S. Pat. No. 5,479,638). The deletion cycle means that a memory cell of a block to be deleted is deleted after programming (data writing) is complete. According to this memory system, the execution of the deletion cycle is prevented by programming update data in an unused part of a memory block without overwriting changing target data itself with the update data unlike a hard disk. However, the programmed memory block is periodically cleaned up. To execute the process, the memory system includes a flag indicating that a sector of each memory block is unused, used, or invalid, and a map for direct correspondence between an eternally provided logical address and a physical address of the memory block.